81^2 = 18 ^2 + 80^ 6561 = 324 + 6400 6561 = 6724 <span>It is not a right angle, since 18^2 + 80^2 does not equal 81^2. So, Using </span><span>The law of cosines: </span>The largest angle C is opposite to the side c. Therefore, <span>c^2 = a^2 + b^2 -2ab cosC </span> <span>Here, c^2 < (a^2 + b^2) cosC must be positive which makes C an acute angle.</span>